The present invention relates to a magnetic recording disk comprising a non-magnetic substrate and two or more firmly adhering magnetizable layers, consisting in particular of an anisotropic magnetic material finely dispersed in an organic binder, applied to one or both sides of the substrate.
The recording of information, especially in the field of data processing, in concentric tracks on rigid or flexible magnetic disks is known (Great Britain Pat. No. 1,416,495).
Such recording media are usually produced by applying a dispersion of a particulate anisotropic magnetic material, e.g. acicular gamma-iron(III) oxide, in a solution of an organic binder, also containing suitable additives, to the substrate. In the case of rigid disk memories the dispersion is usually applied to aluminum disks by the spin-coating process (U.S. Pat. No. 3,198,657), whilst flexible magnetic disks are punched out of a web of base film coated with the dispersion (Great Britain Pat. No. 1,416,495). In these recording media, the anisotropic magnetic particles are either unoriented or circularly oriented (German Laid-Open Application DOS No. 2,304,311) and Great Britain Pat. No. 1,416,495.
As is well known, alignment of the anisotropic magnetic particles in the recording layer increases the remanent magnetization in the direction of orientation (see U.S. Pat. No. 4,277,809 at column 1, lines 14 through 46), as a result of which a higher output signal level in the recording direction is achieved. This is why, in the case of magnetic recording tapes, the particles are oriented in the envisaged recording direction.
However, when an unoriented magnetic coating is compared with an oriented coating of the same thickness, it is found that the ratio of the recorded signal at high frequencies, i.e. at high recording densities, to that at low frequencies, i.e. at low recording densities, is higher in the case of the un-oriented coating. This effect can only be compensated for by reducing the thickness of the coating. It has also been proposed in U.S. Pat. Nos. 3,185,775 and 3,052,567, and Austrian Pat. No. 224,357 to orient the anisotropic particles, within a dual-layer magnetic coating, in such a way that their easy axis is substantially parallel to the field lines in front of the recording head (see also U.S. Pat. No. 4,287,544 at column 1, lines 16 through 18). This means that the particles in the layer which is remote from the magnetic head are oriented in the recording direction and parallel to the base, whilst the particles in the upper layer are virtually at right angles to the base. With this method it was possible to substantially broaden the frequency range of magnetic recording media for analog recording, especially in the audio sector. However, the same considerations do not apply to the recording of digital information. A decrease in the output signal level is undesirable here; if this decrease is brought about by a reduction in the thickness of the coating, there is usually a substantial increase in the number of defects in the coating. Furthermore, in the case of thin coatings, the surfaces are difficult to process in the manner usually used in the case of magnetic disks to achieve the requisite planeness and slight surface roughness, without the occurrence of flaws in the coating (see generally U.S. Pat. No. 4,277,809 at column 1, lines 18 through 38). On the other hand, a non-oriented magnetic layer has the advantage over an oriented layer of the same thickness that it exhibits much better frequency response characteristics. However, a disadvantage is the very low read signal, which causes a worsening of the signal/noise ratio by a factor of about 2. This in turn means that the number of errors increases, since the noise pulses produce phase errors in the read signal. This is particularly critical in the case of the innermost tracks of a magnetic recording disk, where the recording density is highest and the relative speed between the head and the disk is lowest, i.e. the read signal, especially a high-frequency read signal, is weakest.